Asthma is a leading cause of childhood and adult morbidity with an estimated 300 million sufferers worldwide. The incidence has risen dramatically in recent decades, with most asthmatics diagnosed by 6 years of age (1-7). This increase is linked to changes in environmental factors affecting the immune system in early life (8-10). One potential factor is vitamin D, acquired primarily via exposure to sunlight. A high prevalence of vitamin D insufficiency exists worldwide (11, 12), associated with autoimmunity, cancer (12, 13) and poor pulmonary function (14). Our collaborators in Boston, MA and Aberdeen, Scotland showed that higher maternal dietary intake of vitamin D during pregnancy is associated with a significantly lower risk for recurrent wheezing in 3- and 5-year old children (15) (16). These data form the basis for a recently funded clinical trial, involving 870 women, to study the effects of high dose vitamin D supplementation in pregnancy on the incidence of wheeze and respiratory disease in the offspring (NIH Grant number: U01HL091528, co-PIs: Litonjua & Weiss). Vitamin D is an important modulator of immunity. The active form of vitamin D enhances the frequency of two distinct populations of T regulatory cells (Treg), IL-10 secreting (17) and Foxp3+ Treg cells (18). These Treg maintain immune homeostasis in the respiratory environment (19). Their functional impairment is seen in allergy and asthma in young children and cord blood (19-24). Vitamin D may have further benefit in asthma by enhancing responsiveness of corticosteroids (25). We hypothesize that maternal vitamin D status influences immunity in the neonate, specifically the frequency and function of Foxp3+Treg and IL-10-Treg cells. We will investigate: 1. Does vitamin D promote functional Foxp3+ and IL-10+ regulatory T cells in cord blood in vitro? These studies will utilize cord blood from healthy, term deliveries to identify the capacity of active vitamin D, calcitriol, to induce IL-10-Treg vs. Foxp3+Treg; the role of antigen presenting cells; the suppressive capacity of and biomarkers specific to calcitriol-induced Foxp3+Treg vs. IL-10-Treg. 2. Do low maternal levels of vitamin D lead to impaired immune development in the neonate, and an inappropriate balance of regulatory to effector T cell populations? An ethically approved clinical trial (NIH Grant Number: U01HL091528) will provide cord blood mononuclear cell samples from babies of mothers receiving 400IU (low) versus 4400IU vitamin D (high) supplementation during pregnancy to study how maternal vitamin D status alters immune cell composition, including effector and regulatory T cell frequencies; allergen-induced cytokine production; the inducibility of Foxp3+Treg versus IL-10-Treg; and the responsiveness to corticosteroids for induction of IL-10. 3. Do immunological parameters predict clinical outcome related to vitamin D status in utero?